A number of actuator devices typically referred to depending on the actuation direction, such as pin pullers or pin pushers, which are usually custom-made devices, depending on their application, being formed as a passive system which allows initiating an event by means of displacing a shaft or pin, are known today.
Explosive or pyrotechnic devices, also referred to as drives, which have been widely used for different functions in the aerospace field given their minimum volume/weight ratio, having instantaneous on-demand operation, and requiring little power for supplying them, furthermore being self-contained, are known in the field of these devices. For activation, these devices comprise an initiating material, which is heated until reaching self-ignition. The explosive devices perform functions of releasing, cutting, pressurizing, using in valves, igniting, using in switches and other applications in which mechanical work is required. They further allow long-term storage, are highly reliable, relatively inexpensive and power supply is limited compared with the power they can provide.
However, their drawbacks include the fact that they can only be used once, making the device testing phase complicated given that after each activation the explosive elements must be replaced. On the other hand, these actuators present handling, storage and maintenance problems in addition to containing hazardous and unstable materials, producing pollution and having high impact and vibration levels given their explosive and mechanical nature, making them incompatible in high-precision applications. All these operating and safety characteristics as well as new technology availability issues today question their use in future space missions.
On the other hand, in contrast with the aforementioned devices there are non-explosive actuator devices which have the main advantage that they can usually be reinitiated for reuse and they do not incorporate hazardous materials, although their volume/weight ratio and cost are greater than those of explosive actuator devices. Nevertheless, given their safety conditions, non-explosive actuator devices are widely used today for space applications, such as in satellites for example. However, with respect to pyrotechnic devices, non-explosive actuators do not require being as robust and can usually be simpler devices.
Some of the non-explosive actuator devices existing today are explained below.
Paraffin actuators use the high pressure produced by the volumetric expansion of paraffin when there is a solid phase to liquid phase transition (molten paraffin) to produce mechanical work in the form of the linear displacement of a piston. They are small-sized devices considering the force and the stroke, or displacement of an actuator pin or shaft, which they allow. The work is generated by the heat produced by internal electric resistance elements or even by room temperature gradients for the purpose of producing said phase change, no they are extremely sensitive to environmental conditions and their calibration can be complex in precision applications, making them more expensive.
Actuators commonly referred to as burn wire are based on the rapid heating of a wire which in turn releases a spring-operated mechanism. When a current is applied to the wire, the resistance to heating produces an increase of the temperature of the wire until it approaches melting and it breaks, such that by selecting the spring and other elements it is controlled that it breaks with a specific electric current. The main drawback of these devices is that the current for melting a long wire is very high, usually exceeding the available capacity in aircraft power systems in addition to it not being a reusable device.
Electromagnetic actuators have an optimal ratio between the force they can provide and their mass, consuming energy only during their actuation, in addition to allowing multiple actuations and having a modular construction; however, despite allowing a long stroke, they have little force.
Unlike electromagnetic actuators, piezoelectric actuators allow a short stroke and have a lot of force. These actuators are very fast, require high voltages and are very precise, therefore they have high repeatability, returning to their rest position when the electric action activating them no longer exists. Furthermore, these actuators are operative in a broad temperature range, which allows their use primarily in aerospace applications. However, even though these actuators can develop high stresses, of the order of 10-40 MPa, their displacement is relatively low, of the order of nanometers, greatly limiting their use in specific applications.
Finally, there are actuators commonly known by their abbreviations, such as SMA (i.e., shape memory alloy). An example of such actuators is described in U.S. Pat. No. 5,771,742-A, where said SMA is used as an activating element to trigger the operation of the movable element or pin of the device. Compared with the remaining actuators, actuator devices incorporating SMA have better performance in terms of force considering their weight and volume. However, actuators based on SMA technology which are currently on the market have limitations for actuation at high temperatures.
Among non-explosive actuator devices, in addition to the drawbacks and limitations described in the preceding paragraphs for each type of device, there is a series of limitations concerning their field of application. In this sense, some devices are restricted to very limited force and movement ranges or consume a great deal of power. On the other hand, there are devices having large dimensions, which also limit their application. Others, such as electromagnetic devices, have considerable weight. There are those which generate and are sensitive to electromagnetic noise, making their application in noisy environments or in environments where electromagnetic disturbances should not be generated difficult, whereas some are only operative in a temperature range limited to environments under 90° C., which prevents their use in the space, aeronautic or automotive sectors, where higher operating temperatures are often required.
There is no device today in which different variables are simultaneously optimized relating to volume, degree of technical complexity, cost, precision, reliability, versatility concerning its applications and actuation capacity or available stroke. Likewise, one of the main limitations of devices today is the operating temperature range, which has maximum temperature values that could be low for certain applications, mainly in the aerospace field.